the argument is that it is necessary to control the incentive for production because production of child pornography is where child abuse comes from.
put that way it seems reasonably straightforward but in actuality there are many steps removed from the actual harm to CP. first the person involved has to not consent (by law, although i have a different take on what it is about consent that is actually important), second there must be actual harm rather than theoretical harm in the actions taken. third the act must have been recorded when it would not have been but for the incentive provided by distribution. fourth, it must actually be distributed. fifth that distribution must have been intentional rather than accidental or incidental. sixth, the acquisition of the CP must have actually provided some motivation or incentive for its production, and finally, the acquisition must have been done intentionally with at least reckless disregard for the motivational consequences. NONE of which addresses the issue that possession itself does not prima facia lead to ANY of that.
backing up to the first step: what matters isn't whether someone consents but whether they WOULD have consented if they had the ability to do so. this is the role that guardians play, to make that decision. ex. can a mentally retarded person be consigned to a life of non-sexuality just because they may never have the ability to properly consent to sexual activity? should any person trying to become sexual with them be arrested for rape? they are sexual beings, as are children, and their needs are not addressed by whether or not they DO consent, but by whether or not they WOULD consent.